In a UASB reactor, the cells are transported by one or a combination of the following mechanisms: diffusion(Brownian motion), advective (convective) transport by

fluid flow, gas flotation or sedimentation. The initial adsorption can take place after a collision between the cells and the substratum. The substratum can either be other cells or bacterial aggregates present in the sludge or organic or inorganic materials that can function as growth nuclei. The initial adsorption can be approximately described by the DLVO theory, presented by Derjaguin, Landau, Verwey and Overbeek between 1940 and 1950, with the aim of explaining colloid stability. This theory can explain and/or predict microbial adhesion using calcula-tions of adhesion free energy changes. By using this theory, the assumption is made that bacteria behave as inert particles and that bacterial adhesion can be understood by a physico-chemical approach. The DLVO theory postulates that the total long-range interaction over a distance of more than 1 nm is a summation of Van der Waals and Coulomb (electro-static) interactions. According to this theory, three different situations can occur:

1. a repulsion when electrostatic interactions dominate;

2. a strong irreversible attraction when Van der Waals forces are dominant (primary minimum);

3.a weak, reversible attraction when cells are located a certain distance from each other (secondary minimum).